Any temperate above 120 F is going to burn your skin, which will blister and kill it. Maybe even lower, if you have sensitive skin in some areas. Even 110 F for 25 minutes will damage your skin, but it *should* recover. For most people, a temperature above 106 F is painful.

If your skin becomes red or flushed, the temperature is too high! Best for most people is between 95 and 105 F, but some people do better with cooler.